NIP - Notice Of Intended Prosecution.
This is that all important bit that people keep asking me about. It pertains to the 14 day rule that surrounds service of an NIP. As with all traffic offences in the UK, an NIP is required for prosecution to proceed. If you're stopped at the scene however (for example, by a radar-gun-toting traffic policeman), then a verbal NIP is sufficient. Otherwise it must be in writing. So theoretically, if you've not heard anything after 3 weeks, you got away with it. Not, of course, that the police don't get the calculations wrong from time to time - there's no better way to knock the smile off a prosecutors face than to ask if they can prove service of an NIP.
In order to remove some of the mystery from this area - or cast it into a murky grey area depending on your view of things, here are the relevant excerpts from Butterworth's Police Law (Butterworth's Law; ISBN 0406981469). Thanks to Simon Hepworth for help with this information.